Bonding in clusters. Part 12. A reinterpretation of the bonding of arachno-boranes

Abstract

A series of MNDO (minimum neglect of differential overlap) calculations have been used to assess the standard relationship between arachno- and closo-boranes, namely that the arachno-boranes are related both structurally and in their bonding to a closo-borane with two adjacent boron atoms deleted. It is found that there is no evidence from the calculations to support this description but that a far more valuable correlation is that between arachno- and nido-boranes with the same number of boron atoms. These are inter-related by chemical reduction and oxidation. One of the n+ 2 cluster orbitals of the corresponding nido compound is destroyed by the reduction, and two additional exo-hydrogen bonds (or lone pairs in the anions) are created. The arachno-borane B_nH_n+6 has n+ 1 cluster-bonding orbitals and not n+ 3 as proposed in the standard description.